One example of a solids-liquid separating process is a dewatering process for municipal sludges. The dewatering of low consistency sludge is often carried out using a dewatering machine such as, for example, a screening table or filter belt. Prior to introducing the sludge to the head box of the dewatering machine, the sludge is admixed with a polymeric flocculent. The flocculent significantly improves the dewatering of the dewatering machine.
The dosage or amount of the flocculent admixed with the sludge is an important parameter for technological reasons, since the highest dry matter content of the sludge is only attainable with correct dosage. In addition, it is desirable to avoid using excess amounts of the flocculent for economic considerations since the flocculent usually is very expensive. Both over-flocculating as well as under-flocculating reduce the efficiency of the solids-liquid separation process. Moreover, for economic reasons, it is necessary to optimize dosing in order to avoid unnecessary over-flocculation.
When manually adjusting the amount of flocculent added to the sludge as is frequently practiced, high demands are made on the operating personnel both with regard to estimating the correct flocculation state as well as with regard to concentration. This is aggravated by the fact that the effect of adjusting the amount of flocculent does not become apparent for several minutes after addition, because the flocculent is placed upstream of the head box sufficiently far to provide adequate time for flocculation of the sludge prior to its entering the screen. Thus, there is typically a long lag time between changes in the addition of the flocculent and the affect the change has on the dewatering.
One example of a known automatic process for controlling the process conditions in a dewatering process is disclosed in EPO 316,997 B1. The disclosed process detects light reflected form a moist surface of the material being filtered by photodetectors, compares the value with a desired known value and in accordance therewith, regulates the amount of flocculent added to the material. This process has the disadvantage that for each new sludge composition, a new desired known value must be prescribed. In addition, where the input conditions of the dewatering process fluctuate greatly, such as in the sludge composition, the plant will function inadequately and inefficiently.
The above-noted methods and apparatus for controlling and adjusting the operating parameters of liquid separation apparatus do not continuously operate the apparatus at peak or optimum performance. Accordingly, there is a continuing need in the industry for improved methods and apparatus for optimizing the performance of a liquid separation device.